The 3rd-Generation Partnership Project (3GPP) has developed a set of standards for a third-generation (3G) wireless communications system referred to as the Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN) or the Long-Term Evolution (LTE) system. Parts of the LTE specifications define a medium access control (MAC) layer that uses a multi-process, stop-and-wait, hybrid automatic repeat-request (HARQ) protocol for data transmission between the LTE base station (evolved Node-B, or eNodeB) and a user mobile device (user equipment, or UE).
In LTE and other systems that use multi-process HARQ, each data packet sent from a transmitter to a corresponding receiver is associated with an active HARQ process. The receiving entity provides feedback indicating successful or unsuccessful reception of the data associated with a process and, based on the received feedback, the transmitting entity attempts to perform a retransmission. In LTE systems in particular, the base station transmits control signaling to provide information about the successful reception of (or failure to receive) the data associated with an uplink HARQ process; this control signaling is processed by the physical layer entity at the UE and provided to the MAC layer. If the control signaling indicates a failure to receive the data associated with a given process, the MAC layer in turn delivers a retransmission packet to the physical layer.
In LTE systems, the base station generally explicitly assigns transmission resources (one or more OFDM resource blocks, for one or multiple time intervals) to a terminal for both uplink and downlink transmissions, and determines the transmission format (modulation and coding scheme) to be used. For LTE uplink transmissions, HARQ retransmissions may be non-adaptive (i.e., using the same transmission resources and transmission format as used for the original transmission), or adaptive (i.e., one or both of the transmission resources and transmission format are explicitly modified by the base station). The specifics of each retransmission are controlled based on the physical layer signaling from the base station to the mobile terminal.
In more detail, an LTE mobile terminal receives uplink grant information via the Physical Downlink Control Channel (PDCCH). The grant message specifies the HARQ process ID, type of transmission (new/retransmission), redundancy version, etc. ACK/NACK messages corresponding to the HARQ processes are sent via the Physical HARQ Indicator Channel (PHICH). However, an explicit grant for a given process (received on the PDCCH) overrides the ACK/NACK messages, so that the ACK/NACK status is ignored if a PDCCH grant is received. As noted above, a PDCCH grant explicitly specifies the transmission resources and transmission format, and thus may specify an adaptive retransmission for a given HARQ process.
If a PDCCH message for a given process is not received, then the HARQ feedback (ACK/NACK) is considered. In particular, if a NACK is received then a synchronous non-adaptive retransmission is scheduled for the next transmission time interval assigned to that process. If an ACK is received, then no non-adaptive retransmission is planned. However, the buffered data for that HARQ process is preserved until a PDCCH grant for that process is eventually received. As a result, a subsequent grant may request a re-transmission of the HARQ process data (even if an ACK was previously received), or may grant resources for a transmission of new data for the HARQ process. Those skilled in the art will appreciate that the former grant can be viewed as an implicit NACK, while the latter is an implicit ACK. The skilled practitioner will further appreciate that this procedure allows for rapid recovery from several signaling error scenarios, such as ACK-to-NACK and NACK-to-ACK reception errors.
LTE base stations may occasionally request that the mobile terminal transmit physical layer information, such as channel quality data. The radio link resources (e.g., in time and frequency) for transmitting such additional information elements may be pre-configured or allocated dynamically, e.g., on the Physical Uplink Control Channel (PUCCH). The base station may also determine whether the terminal may send only the requested information element(s) in a given transmission time interval, or whether the mobile terminal may multiplex the information elements with other data, such as the data from a current stop-and-wait HARQ process. The LTE base station may signal this decision to the terminal using a dedicated control indicator that is associated with other physical layer control information provided by the base station to the terminal. For convenience, this control indicator is called a CQI-only indicator in the discussion that follows, although those skilled in the art will appreciate that this indicator (or other flag or indicator) may be used to indicate the exclusive transmission of other physical layer information elements, such as a power headroom report.
If the base station indicates to the terminal that the requested information element must not be multiplexed with data, the terminal must suspend an active process if that process has data pending for retransmission, e.g., via a non-adaptive retransmission. Furthermore, a mechanism is needed for resuming the pending process at the next appropriate transmission opportunity. According to a conventional solution, upon receiving an indication that only requested information elements may be transmitted during a transmission time interval corresponding to a pending HARQ retransmission, the physical layer of the mobile terminal indicates a positive feedback (ACK) to the LTE MAC layer, which in turn suspends the HARQ process. As noted above, the HARQ process buffer is retained even upon receipt of an ACK, and is flushed only upon receipt of an uplink grant indicating that new data should be sent for that process. Thus, a HARQ process suspended in the manner described above can be resumed by an adaptive uplink grant indicating a retransmission, i.e., having the same new-data indicator (NDI) value.
This particular mechanism for suspending a HARQ process upon receipt of a CQI-only indicator may be implemented in two ways. First, the ACK could be sent by the eNodeB over the PHICH and forwarded by the physical layer to the MAC layer. Alternatively, the ACK message could be locally generated by the physical layer upon reception of an uplink grant for a CQI-only transmission. This latter approach avoids conflicting behavior of MAC and physical layer in case of errors in receiving the control signaling.
However, an obvious drawback of suspending the HARQ process by indicating an ACK is that this approach requires an adaptive uplink grant to be sent on PDCCH to resume the uplink transmission. This costs scarce layer 1/layer 2 control signaling resources, and increases error probabilities.
Another proposed approach is to configure the base station so that it does not prohibit multiplexing of data from a stop-and-wait process if there is a pending retransmission. In other words, the base station avoids sending a CQI-only indicator if there is a pending non-adaptive retransmission for the corresponding HARQ process. One drawback of this approach is that the transmission delay of the information elements may be increased.